The present invention relates to a process for the preparation of polycarbonates. More particularly, the present invention allows for the preparation of polycarbonates via an integrated process utilizing as reactants a bisphenol and phosgene.
In U.S. Pat. No. 4,452,968 an integrated process for the preparation of aromatic polycarbonates was disclosed wherein a methyl acetate by-product formed in the transesterification of a bisphenol diacetate with a dialkyl carbonate was recycled by heating the alkyl ester to form a ketene and subsequently such ketene was reconverted by reaction with a bisphenol to form additional quantities of the bisphenol diacetate. For the teachings contained therein the above identified patent is incorporated herein by reference thereto.
Preparation of a ketene intermediate has proven costly and inefficient due to the high temperatures involved and the fact that a dialkyl carbonate intermediate is formed in the process. Recoveries on the order of only about 80% of theoretical or less are typical, leading to the need to develop a more efficient means of generating polycarbonate resins.
Another integrated process for preparing polycarbonates is disclosed in U.S. Pat. No. 4,954,613. In this process a dialkyl carbonate is reacted with a bisphenol dicarboxylate to prepare the polycarbonate oligomer. The alkyl ester byproduct is carbonylated to form an anhydride, typically acetic anhydride, which is reacted with a bisphenol to regenerate the bisphenol dicarboxylate. This step liberates a coproduct carboxylic acid, typically acetic acid.
The generation of the coproduct is undesired since the process requires either use or disposal of the coproduct. Often the coproduct must be purified before use or sale thereby adding further complexity and cost to the total process. In practice the producer is rarely able to maximize recovered costs since the byproduct volumes are determined in fixed ratio to generation of the desired product and not according to a separate marketing plan.